Human skin consists of two layers, the uppermost of which is the epidermis. The epidermis encompasses many different cell types, including melanocytes and keratinocytes. Melanocytes are specialized cells in the basal layer of the epidermis which synthesize melanin; the melanin is then packaged into melanosomes and then transported into keratinocytes.
It has been known for centuries that exposure of skin to the sun results in tanning, the skin's major form of endogenous protection against subsequent skin damage from ultraviolet (UV) irradiation. Melanin, a polymer which serves as a filter with absorbance within the UV range, provides photoprotection for the individual. The peak action spectrum for this phenomenon is in the UV-B range, 290-305 nm; various morphologic and enzymatic changes occur at the cellular level in epidermal melanocytes in response to UV irradiation. Proteins and nucleic acids of the epidermis absorb UV-B rays, causing the production of thymine dimers, which are known to be formed by UV irradiation of nuclear DNA and to be excised from the DNA strand by the action of highly specific enzymes, including endonucleases. If not removed, these dimers can stall DNA replication forks generating regions of single-stranded DNA. Failure to remove thymine dimers and other DNA mutations in the genome may lead to somatic mutations resulting in carcinogenesis.
In bacteria it is known that the DNA fragments released from stalled replication forks can interact with nuclear proteins which then regulate the expression of specific genes in the DNA as part of the organism's SOS response to UV damage. Bacteria do not tan, but tanning might reasonably be considered part of the analogous SOS response in mammalian skin. The precise stimulus for UV-induced tanning, however, remains unknown.